Re: [PATCH rcu 04/12] srcu: Bit manipulation changes for additional reader flavor

[Neeraj Upadhyay]

On 10/14/2024 10:21 PM, Paul E. McKenney wrote:
> On Mon, Oct 14, 2024 at 02:42:33PM +0530, Neeraj Upadhyay wrote:
>> On 10/9/2024 11:37 PM, Paul E. McKenney wrote:
>>> Currently, there are only two flavors of readers, normal and NMI-safe.
>>> Very straightforward state updates suffice to check for erroneous
>>> mixing of reader flavors on a given srcu_struct structure.  This commit
>>> upgrades the checking in preparation for the addition of light-weight
>>> (as in memory-barrier-free) readers.
>>>
>>> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
>>> Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
>>> Cc: Andrii Nakryiko <andrii@xxxxxxxxxx>
>>> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>>> Cc: Kent Overstreet <kent.overstreet@xxxxxxxxx>
>>> Cc: <bpf@xxxxxxxxxxxxxxx>
>>> ---
>>>  kernel/rcu/srcutree.c | 7 ++++---
>>>  1 file changed, 4 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
>>> index 18f2eae5e14bd..abe55777c4335 100644
>>> --- a/kernel/rcu/srcutree.c
>>> +++ b/kernel/rcu/srcutree.c
>>> @@ -462,7 +462,7 @@ static unsigned long srcu_readers_unlock_idx(struct srcu_struct *ssp, int idx)
>>>  		if (IS_ENABLED(CONFIG_PROVE_RCU))
>>>  			mask = mask | READ_ONCE(cpuc->srcu_reader_flavor);
>>>  	}
>>> -	WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && (mask & (mask >> 1)),
>>> +	WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && (mask & (mask - 1)),
>>>  		  "Mixed NMI-safe readers for srcu_struct at %ps.\n", ssp);
>>>  	return sum;
>>>  }
>>> @@ -712,8 +712,9 @@ void srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor)
>>>  	sdp = raw_cpu_ptr(ssp->sda);
>>>  	old_reader_flavor_mask = READ_ONCE(sdp->srcu_reader_flavor);
>>>  	if (!old_reader_flavor_mask) {
>>> -		WRITE_ONCE(sdp->srcu_reader_flavor, reader_flavor_mask);
>>> -		return;
>>> +		old_reader_flavor_mask = cmpxchg(&sdp->srcu_reader_flavor, 0, reader_flavor_mask);
>>
>> This looks to be separate independent fix?
> 
> I would say that it is part of the upgrade.  The old logic worked if there
> are only two flavors, but the cmpxchg() is required for more than two.
> 

Ok, I need to check more to understand why it is not required when we
have only two flavors.


- Neeraj

> 							Thanx, Paul
> 
>> - Neeraj
>>
>>> +		if (!old_reader_flavor_mask)
>>> +			return;
>>>  	}
>>>  	WARN_ONCE(old_reader_flavor_mask != reader_flavor_mask, "CPU %d old state %d new state %d\n", sdp->cpu, old_reader_flavor_mask, reader_flavor_mask);
>>>  }
>>